An image forming lens for forming an image from an infinity-designed objective lens is developed as an image forming lens of which various aberrations are corrected (refer to, e.g., Japanese Patent Publication No. 2521959 or Japanese Patent Laid-Open Publication No. 05-113540). Over the recent years, a fluorescent observation has been actively utilized for a biological research. Especially, an actively used technique is a technique of observing simultaneously behaviors of a plurality of proteins by using fluorescent makers having a plurality of different fluorescent wavelengths. On this occasion, if the fluorescent wavelengths of the fluorescent makers are close to each other, wavelength ranges thereof are overlapped with each other, and it can not be determined from which fluorescent marker the fluorescence is emitted. It is therefore desirable that the fluorescent wavelengths of the individual fluorescent markers are distanced to some extent. As a result, the fluorescent makers having a variety of fluorescent wavelengths ranging from the near infrared to the near ultraviolet are employed, and an optical system of a microscope for observing the fluorescent markers is requested to have flexibility to a wide wavelength range. Moreover, a two-photon-excited luminescence observation contrived to reduce a damage to a cell by using infrared-rays and a luminescence observation using near ultraviolet rays for improving a resolution come to be carried out, and the microscope is requested to have an optical system which corrects aberrations in wave ranges hereof and exhibits a high transmittance, and this request is given to an image forming lens as well as to the objective lens.
The hitherto-used image forming lens has, however, a problem that a chromatic aberration is not sufficiently corrected, and further transmittance is not taken into consideration.